Formation and migration of charged native point defects inMgH2: First-principles calculations

Abstract

Using first-principles calculations we have investigated the possible native point defects in bulk ${\text{MgH}}_{2}$. Due to the interest in this material for hydrogen storage, we have paid particular attention to hydrogen-related defects that are likely to be involved in the absorption and release kinetics of hydrogen. We have considered neutral and charged defects and calculated formation energies as a function of Fermi-level position and hydrogen chemical potential. In the absence of impurities, we find that under extreme H-poor conditions the lowest-energy defects are positively and negatively charged hydrogen vacancies (${V}_{\text{H}}^{+}$ and ${V}_{\text{H}}^{\ensuremath{-}}$). Under extreme H-rich conditions, the lowest-energy defects are ${V}_{\text{H}}^{+}$, negatively charged hydrogen interstitials $({\text{H}}_{i}^{\ensuremath{-}})$, and negatively charged Mg vacancies ${V}_{\text{Mg}}^{2\ensuremath{-}}$. The defects are characterized by unusually large local structural rearrangements. The hydrogen-related defects are also highly mobile, with a lowest migration barrier of less than 0.10 eV for ${\text{H}}_{i}^{\ensuremath{-}}$ and ${\text{H}}_{2i}$, and a highest barrier of 0.63 eV for ${V}_{\text{H}}^{\ensuremath{-}}$. By combining the calculated formation energies with migration barriers, we find that the lowest activation energy for self-diffusion is about 1.48 eV under H-poor conditions. The consequences of these results for the hydrogenation and dehydrogenation kinetics are discussed.